Pair of progressive ophthamlic lenses

ABSTRACT

A pair of progressive ophthalmic lenses ( 1, 2 ) meets special conditions for improving binocular vision of a wearer, while avoiding discomfort for peripheral vision. A first one of the conditions relates to width values of far vision fields and/or proximate vision fields, for indicating that the fields are different enough in width between both lenses. A second one of the conditions sets a maximum value for the relative difference in mean refractive power gradient between both lenses.

TECHNICAL FIELD OF THE INVENTION

The invention relates to a pair of progressive ophthalmic lenses, and toa method for providing such lens pair to a wearer.

In the context of the present invention, a pair of ophthalmic lensesmeans two eyeglasses which are intended to be provided to one and samewearer when both eyeglasses are mounted into one spectacle frame. Eachlens of the pair is therefore dedicated to correcting both an ametropiaand a presbyopia of one of the wearer's eyes in accordance to anophthalmic prescription obtained for this eye. Visions of both right andleft eyes are thus corrected simultaneously.

The invention specifically relates to provide improved binocular visionto the wearer, by selecting appropriately the progressive lenses to bepaired.

BACKGROUND OF THE INVENTION

Progressive ophthalmic lenses allow sharp vision of objects which arelocated at varying distances in front of the lens wearer, the wearerbeing presbyopic. For this purpose, each progressive ophthalmic lens hasa mean refractive power which varies along a meridian line of the lens.Here, the meridian line corresponds to the main gaze direction used bythe wearer to see at varying distances. The vision distances may bedefined by using an ergorama. As a general rule, the vision distancedecreases from more than 2 meters for a far vision gaze direction, toabout 40 centimeters for a proximate vision gaze direction which isoriented below the far vision gaze direction.

But out of the meridian line, a progressive ophthalmic lens has meanrefractive power values and resultant astigmatism values which are notequal to those on the meridian line at constant height value for thegaze direction lowering angle. As a consequence, the optical powerswhich are actually produced by each lens do not match exactly theprescription out of the meridian line and out of lens zones dedicatedrespectively to far vision and proximate vision. Many efforts have beenspent for years to increase the far- and/or proximate vision areas incombination with reducing the correction deviations out of these areasand the meridian line. But these improvements relate in optimizing eachlens separately.

In addition, it has been discovered that the wearer's binocular visioncan be improved by selecting appropriately both lenses which pertain toone and same pair, each with a different lens design. For example,document FR 2 704 327 proposes selecting one of the ophthalmicprogressive lenses of the pair with a large far vision zone, and theother ophthalmic progressive lens of the pair with a large proximatevision zone. As a consequence, the lens with large far vision zone has areduced proximate vision zone, and the one with large proximate visionzone has a reduced far vision zone. But binocular vision allows thewearer to select the one of his eyes which has sharper vision for eachbinocular gaze direction. Thus, each of the largest far- and proximatevision zones between both lenses appears to be actually effective forproviding sharp binocular vision, whereas the smallest far- andproximate vision zones are useful for providing stereoscopic visionperception.

U.S. Pat. No. 7,083,277 discloses another example of lens pair with bothlenses being different in far- and proximate vision field widths.

However, vision discomfort may still arise when both lenses of the pairare thus selected with different designs. In particular, such discomfortrelates to peripheral gaze directions with respect to the meridian linesof the lenses.

Starting from this situation, an object of the present inventionconsists in improving binocular vision of a wearer of ophthalmicprogressive lenses, while avoiding discomfort for peripheral vision.

SUMMARY OF THE INVENTION

For meeting this object or others, the present invention proposes a pairof progressive ophthalmic lenses, in which each lens has a prescribedaddition ADD which is common to both lenses of the pair and, in as-wornconditions of the lens by a wearer, a meridian line, a nasal side and atemporal side on either side of the meridian line, respectively, afitting cross, a prism reference point, a proximate vision gazedirection, a mean refractive power PPO_(α,β) and a module of resultingastigmatism ASR_(α,β) for each gaze direction through the lens andcorresponding to a lowering angle α and an azimuth angle β, relating toa cyclopean coordinate system.

For each progressive lens of the pair, a mean refractive power deviationDPPO_(α,β) is defined for each gaze direction through this lens asfollows, separately from the other lens of the pair:DPPO _(α,β) =PPO _(α,β) −PPO _(α,βmeridian),where PPO_(α,βmeridian) is the mean refractive power on the meridianline for the same lowering angle value α than the gaze directionconsidered. The meridian line ML corresponds to the main gaze directionused by the wearer to see at varying distances. Then and again for eachlens separately, a total deviation P_(α,β) is further defined for eachgaze direction through this lens as:P _(α,β) =m*(DPPO _(α,β))^(a) +n*(ASR _(α,β))^(b),where m, n, a and b are parameters such that m≧0, n≧0, (m, n)≠(0, 0),0<a≦2 and 0<b≦2. For each lens, such total deviation quantifies thedifference between the actual optical power which is produced at a givendirection and corresponding optical powers selected from the meridianline of the lens for the same height. Thus, the m-, n-, a- andb-parameters are selected initially for allowing the total deviationP_(α,β) to be calculated for each gaze direction. The values selectedfor these m-, n-, a- and b-parameters are constant for all featuresrelating to the lens pair considered. In particular, they are constantwhatever the gaze direction for which the total deviation is calculated.

A first width value is also defined for each lens separately as theangular distance between first and second gaze directions respectivelyon the nasal and temporal sides of the lens, both first and second gazedirections with lowering angle α equal to that of a gaze directionthrough the fitting cross, and each where the total deviation P_(α,β)satisfies:P _(α,β) =m*(ADD/8)^(a) +n*(ADD/4)^(b).Such first width value quantifies the horizontal spread of the farvision area of each lens of the pair, from nasal side to temporal side.Thus, m*(ADD/8)^(a)+n*(ADD/4)^(b) is a reference value for the totaldeviation P_(α,β), which is used for defining the first width value,when the total deviation P_(α,β) is calculated as above-indicated usingthe selected values for the m-, n-, a- and b-parameters, for any gazedirection.

A second width value is also defined for each lens separately as theangular distance between third and fourth gaze directions respectivelyon the nasal and temporal sides of the lens, both third and fourth gazedirections with lowering angle α equal to that of the proximate visiongaze direction, and each where the total deviation P_(α,β) satisfies:P _(α,β) =m*(ADD/4)^(a) +n*(ADD/2)^(b).Such second width value similarly quantifies the horizontal spread ofthe proximate vision area of each lens of the pair, from nasal side totemporal side. So m*(ADD/4)^(a)+n*(ADD/2)^(b) is the reference value forthe total deviation P_(α,β), which is used for defining the second widthvalue. It is different from the reference value used for the first widthvalue.

A total horizontal width ratio is further defined for the pair of lensesas the relative difference between the respective first width values ofboth lenses of the pair, or between the respective second width valuesof both lenses of the pair. According to a first feature of theinvention, this total horizontal width ratio is greater than or equal to0.10 in absolute value. This first feature sets a minimum threshold forthe difference in width of far vision zone or proximate vision zonebetween both lenses of the pair. Thus, a large sharp vision zone for oneof the two eyes is provided to the wearer for far- and proximate visionthat provides to the wearer large perceived field of vision for eachvision zone in binocular vision.

Each lens separately also has a maximum mean refractive power gradientproduced for a gaze direction corresponding to a cyclopean gazedirection which is comprised:

-   -   inside a zone centered on to the cyclopean gaze direction        corresponding to gaze directions passing through the PRP and        containing all cyclopean gaze directions (α,β) respecting the        following inequality (|α|²+|β|²)^(1/2)≦40°, and    -   outside a central optical zone; the central optical zone        comprising a main line ML_min (α_(min), β_(min)) of cyclopean        directions for which, for each angle α_(min), the resulting        astigmatism reaches its minimum, the central optical zone being        delimited on either side of the main line by cyclopean gaze        directions whose azimuth angle is equal to β_(min)±5°.

Then, according to a second feature of the invention, a relativedifference in the maximum mean refractive power gradient between bothlenses of the pair is less than or equal to 0.08 in absolute value. Thissecond feature sets a threshold condition relating to the meanrefractive power gradient, for both lenses not to have respectivedesigns in peripheral part which are too different from each other,which can cause discomfort to the wearer. In particular, it avoidsassigning a lens with hard design to one of the wearer's eyes togetherwith another lens with soft design to the other one of the wearer'seyes.

Hence, the invention consists in an advantageous combination of twofeatures which provide an improved binocular vision by using a speciallyselected progressive lens pair, while ensuring comfort in peripheralvision.

In the context of the invention, the cyclopean coordinate system denotesany coordinate system suitable for identifying a gaze direction which isoriginating from one point located between both eyes of the wearer, andwhich is oriented towards an object currently looked at by the wearer.Such direction is called cyclopean gaze direction, and separate gazedirections respectively for both eyes can be derived from the cyclopeangaze direction. But, when characterizing a lens pair according to theinvention without a wearer of these lenses being individuallyidentified, the cyclopean coordinate system to be used is defined by aseparation distance which is equal to 64 mm (millimeter) between bothpupils of a standard wearer, and for a cyclopean eye which is located atmid position between both eyes of this standard wearer.

Also in the context of the invention, the relative difference betweenany two values denotes the ratio of the difference between these valuesover the sum of the same values. Thus, if X and Y denote both valuesconsidered, then their relative difference is Ratio=(X−Y)/(X+Y).

The lenses may further have nasal and temporal half-widths defined inthe following manner for each lens separately:

-   -   first nasal and temporal half-widths defined as the angular        distances from the gaze direction through the fitting cross        respectively to the first gaze direction and to the second gaze        direction; and    -   second nasal and temporal half-widths defined as the angular        distances from the proximate vision gaze direction respectively        to the third gaze direction and to the fourth gaze direction.

Hence, the first nasal half-width relates to the spread of the farvision zone on nasal side, the first temporal half-width relates to thespread of the far vision zone on temporal side, the second nasalhalf-width relates to the spread of the proximate vision zone on nasalside, and the second temporal half-width relates to the spread of theproximate vision zone on temporal side.

According to a first optional improvement of the invention, each lens ofthe pair may have a relative difference between the first nasal andtemporal half-widths of this lens, which is lower than or equal to 0.15in absolute value, or a relative difference between the second nasal andtemporal half-widths of this lens, which is lower than or equal to 0.05in absolute value, or both relative differences lower than or equal to0.15 and 0.05 respectively for the first half-widths and the secondhalf-widths. This optional condition relates to each lens of the pairseparately from the other lens, and means that at least one of the farvision field and the proximate vision field of this lens extends nearlysymmetrically from the meridian line. Thus the wearer benefits of ahomogenous peripheral vision regarding nasal and temporal parts, and canhave similar behaviours towards both right and left sides so as to turnhis head and/or his eyes for viewing sharply an object which is offsetlaterally. Preferably, the relative difference between the first nasaland temporal half-widths may be lower than or equal to 0.12 in absolutevalue, and/or the relative difference between the second nasal andtemporal half-widths may be lower than or equal 0.03 in absolute value,or both, for each lens of the pair.

According to a second optional improvement of the invention, the lenspair may be such that a relative difference in the first width valuebetween both lenses of the pair has an opposite sign to a relativedifference in the second width value between both lenses, for a samelens order in these relative differences respectively for the firstwidth values and the second width values. Hence, optimum combination ofboth lenses can be obtained for providing the wearer with widehorizontal spread of sharp binocular vision, for both far and proximateobjects that are looked at.

The following parameter values may be used for determining either thefirst and second gaze directions, or the third and fourth gazedirections, or all first to fourth gaze directions, in alternativeembodiments of the invention:

-   -   m=0, n=1 and b=1, so that the criterion for determining the        width values is based on the resulting astigmatism only;    -   m=1, n=0 and a=1, so that the criterion for determining the        width values is based on the mean refractive power deviation        only; or    -   m=4, n=2, a=2 and b=2, so that the criterion for determining the        width values is that of Y. Le Grand as disclosed in the article        entitled “Sur le calcul des verres de lunetterie”, Revue        d'Optique, Paris, 1966.

The invention may be advantageously applied when an absolute differencein prescribed far vision mean refractive power between both lenses isless than or equal to 1.0 diopter. Out of these cases, opticalaberrations as power deviation or resulting astigmatism may not be fullymastered as for each vision zones, so that the design differencesbetween both lenses as provided by the present invention can fullyappear.

Similarly, the invention may be advantageously applied when an absolutedifference in prescribed astigmatism value between both lenses is lessthan or equal to 1.0 diopter.

The present invention also proposes a method for providing a pair ofprogressive ophthalmic lenses to an identified wearer, which methodcomprises the following steps:

-   -   /1/ for the wearer to whom both lenses are dedicated:        -   obtaining the prescriptions for both eyes including the            prescribed addition ADD which is common to both lenses of            the pair;        -   obtaining a value for a separation distance between both            pupils of the wearer, and a position of a cyclopean eye for            this wearer; and        -   defining a cyclopean coordinate system for the wearer, based            on the pupil separation distance and the cyclopean eye            position;    -   /2/ for each lens of the pair separately from the other lens,        defining as above the total deviation for each gaze direction        through this lens, and obtaining the first and second width        values, and the maximum mean refractive power gradient for this        lens;    -   /3/ for the lens pair, defining the total horizontal width ratio        and the relative difference in the maximum mean refractive power        gradient between both lenses of the pair;    -   /4/ optimizing at least one lens of the pair so that the total        horizontal width ratio has an absolute value greater than or        equal to 0.10, and the relative difference in the maximum mean        refractive power gradient between both lenses is less than or        equal to 0.08 in absolute value; and    -   /5/ manufacturing both lenses of the pair in accordance with an        optimization result of step /4/.

The values for the m-, n-, a- and b-parameters are selected initiallywhen implementing the invention method, and are kept constant thereafterall along the execution of the method. In particular, these values arere-used without change when calculating the total deviation P_(α,β) forany gaze direction. When the first and second widths are preferred to bebased on the resulting astigmatism only, then the m-parameter equalszero, and the n- and b-parameters both equal unity.

Conveniently, the optimizing process of step /4/ may be implementedusing computer means.

In some implementations of a method of the invention, step /4/ may bebased on data linked to the wearer. These data linked to the wearer maybe obtained themselves before step /4/, for example at a same time asstep /1/ is carried out.

For a lens pair or a method according to the invention, the totalhorizontal width ratio may have an absolute value greater than or equalto 0.15. In combination or independently, the relative difference in themaximum mean refractive power gradient between both lenses may be lessthan or equal to 0.04 in absolute value.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the invention will becomeapparent from the detailed specification below, which is provided onlyfor illustration purpose without inducing any limitation, and based onthe appended drawings now listed:

FIGS. 1a to 1c illustrate optical parameters related to a lens pairaccording to the invention;

FIGS. 2a and 2b are mean refractive power maps respectively for a rightlens and a left lens of a pair according to an embodiment of theinvention; and

FIGS. 2c and 2d are maps of resulting astigmatism respectively for theright lens and the left lens of FIGS. 2a and 2 b.

Same notations used in different ones of these figures have samemeanings.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1a is a perspective view representing both lenses of a pair inas-worn conditions for a wearer, and FIG. 1c is a planar view of thelenses with parameters relating to the cyclopean coordinate system.

In FIG. 1a , reference OR denotes the right eye 10 of the wearerequipped with the lens 1, and reference OL denotes the left eye 20 ofthe same wearer equipped with the lens 2. Hence, both lenses 1 and 2 areworn simultaneously by the wearer to provide him a corrected binocularvision.

The wearing conditions for both lenses 1 and 2 may be the usual oneswhich are known in the art. In particular, both lenses 1 and 2 aremounted together within a spectacle frame (not shown) so that the backsurface of each lens may be located at a distance of about 25.5 mm(millimeter) from the rotation center of the corresponding eye. R_(OR)and R_(OL) denote respectively the rotation centers of the eyes 10 and20. The pantoscopic angle for each lens 1 and 2 may be 8° (degree), witheach lens top edge leaned ahead with respect to the wearer's face. Thewrap angle of each lens 1 and 2 may have a mean value of about 5°, thisangle corresponding to a tilt of each lens about a vertical axis so thatthe temporal edge of the lens is shifted backwards with respect to itsnasal edge.

Progressive lenses comprise micro-markings that have been made mandatoryby a harmonized standard ISO 8980-2 published on 2004 Feb. 1 (ISO8980-2:2004). Temporary markings may also be applied on the surface ofthe lens, indicating positions of control points on the lens, such as acontrol point for far vision, a control point for near vision, a prismreference point PRP and a fitting cross FC for instance. If thetemporary markings are absent or have been erased, it is always possibleto a skilled person to locate the control points on the lens by using amounting chart and the permanent micro-markings. Indeed, item 7.1 c) ofthe standard ISO 8980-2 specifies that the manufacturer's name must bepermanently indicated on the progressive addition lens, and item 8.1 ofthe same standard ISO 8980-2 indicates additional information which mustbe specified on the lens packaging or accompanying documents. It refersto Article 6 of ISO 14889:2003 for reciting this additional informationor information which must be available on request to the lensmanufacturer. This latter comprises lens alignment rules for recoveringthe locations of all reference points and measurement points which maybe non-permanently marked on the lens, including the fitting cross, theprism reference point, the far vision control point, the proximatevision gaze direction (item 6.2 f) of ISO 14889 referring back to item7.2 a) to e) of ISO 8980-2). Therefore, all these points are setunambiguously for any progressive ophthalmic lens, from the initialdesign and the manufacturing of this lens, and cannot be selected in adifferent way when examining this lens later.

The prism reference point PRP is considered here at the midpoint of thestraight segment which connects the circle-like micro-markings. When thelens is mounted in front of the eye, the fitting cross is placed beforethe pupil or before the eye rotation center of the eye for a primarygaze direction. The primary gaze direction corresponds to the situationwhere a wearer is looking straight ahead. In the chosen frame, thefitting cross FC corresponds thus to a lowering angle α of 0° and anazimuth angle β of 0°.

Then, each lens may be mounted within the spectacle frame so that thegaze direction of each eye 10, 20 passes through the fitting cross FC ofthe corresponding lens 1, 2 when the wearer is approximately lookinghorizontally at an object located in front of and far away from him,with the wearer's head itself approximately vertical. G_(FC) denotesthis gaze direction, which is commonly called primary gaze direction.Additionally for each one of the lenses 1, 2, G_(PRP) denotes the gazedirection which passes through the prism reference point PRP of thislens.

N and T denote respectively the nasal side and the temporal side of eachlens 1, 2. Both nasal and temporal sides are separated from each otherby a meridian line ML within each lens. Within each lens, the meridianline ML is the track of the gaze direction of the corresponding eye whenthe wearer is looking successively at objects located at varyingdistances in front of him, as defined by an ergorama. To determine thismeridian line ML by ray tracing, one can consider that for each gazedirection, the wearer is looking at an object located at the distancegiven by the ergorama and contained in the sagittal plane of the wearer.Usually, manufacturers will match this meridian line ML of an ophthalmiclens with approximately the line ML_min containing the gaze directionsthat corresponds to the minimum of resulting astigmatism or the linelocated at almost equal distance from two gaze directions through thelens respectively on the nasal side and the temporal side, with samevalues for the lowering angle and also same values for the resultingastigmatism. According to the monocular coordinate system, each meridianline ML or each line ML_min are contained in a vertical plane above thefitting cross FC, and deflected towards the nasal side N below thefitting cross FC. According to the cyclopean coordinate system, themeridian line ML is contained in a vertical plane for each lenscontaining the fitting cross FC. The line ML_min is called main line foreach lens.

Each lens 1, 2 is selected based on a prescription obtained for thewearer and indicating a prescribed far vision mean refractive powerSPH_M_(VL), a prescribed astigmatism value CYL_(VL) and a prescribedaddition ADD suitable for correcting the ametropia and presbyopia ofeach eye. The mean refractive power is obtained by summing the halfvalue of the prescribed astigmatism to the prescribed sphere:SPH_M_(VL)=SPH_(VL)+CYL_(VL)/2. For the present invention, theprescribed addition ADD is equal for both eyes 10 and 20. Then, the meanrefractive power for each eye for proximate vision is obtained bysumming the prescribed addition ADD to the far vision mean refractivepower SPH_M_(VL) prescribed for the same eye: SPH_M_(VP)=SPH_M_(VL)+ADD.For each lens, the prescribed far vision mean refractive powerSPH_M_(VL) is produced for a gaze direction intersecting the lens frontsurface at a far vision control point VL. For example, this far visioncontrol point VL may be located so that the corresponding gaze directionis 8° above the gaze direction G_(FC) within a common vertical plane.

Also for each lens 1, 2 separately, the calculated proximate vision meanrefractive power SPH_M_(VP) is produced for a gaze direction for whichthe lowering angle is positive, that is to say below the fitting crossFC. For the invention implementations which are described here, aproximate vision gaze direction G_(VP) is defined on the meridian lineML where the mean refractive power has reached 100% of the prescribedaddition ADD, with respect to the prescribed far vision mean refractivepower. Usually, for a regular progressive lens, 85% of the prescribedaddition is reached between 22° and 26° from the fitting cross FC, andgenerally at 24° below the fitting cross FC, and 100% of the prescribedaddition is reached between 34° and 38° from the fitting cross FC, andgenerally at 36° below the fitting cross FC. The intersection of G_(VP)with the front surface is called the proximate vision point VP and doesnot necessarily match the proximate vision control point as indicated bythe manufacturer of the lens.

Any gaze direction G through one of the lenses 1, 2 is originating fromthe rotation center R_(OR) or R_(OL) of the corresponding eye 10 or 20respectively. It is identified using a lowering angle value α_(m) and anazimuth angle value β_(m).

Actually, the lowering angle α_(m) and the azimuth angle β_(m) as justdefined relate to each lens separately, but do not suit for describingbinocular vision and assessing binocular vision performances. So theabove-defined angles α_(m) and β_(m) which relate to monocular visionfor each eye are to be replaced with a common angular system whichdefines the respective gaze directions of both eyes when operatingtogether for binocular vision. FIG. 1b illustrates the definition ofsuch system suitable for binocular vision, as known from WO 2011/061267for example.

The rotation centers R_(OL) and R_(OR) of both eyes are represented asbefore, and a cyclopean eye denoted CE is defined on the straightsegment which connects R_(OL) and R_(OR). The location of the cyclopeaneye CE between both rotation centers R_(OL) and R_(OR) may be determinedin several alternating manners. It may be at the middle position betweenR_(OL) and R_(OR), or shifted along this segment depending on thewearer. In particular, the Man skilled in ophthalmics knows methods fordetermining the location of the cyclopean eye CE from measurementsperformed on the wearer. Such a method may be based on the measurementof a dominance between the right eye 10 and the left eye 20. A dominancerate comprised between −1 and +1 may be determined for the wearer inthis way. The cyclopean eye CE is then superposed with the right eyerotation center R_(OR) when the measured value of the dominance rateequals −1, and it is superposed with the left eye rotation center R_(OL)when the measured dominance rate equals +1. Then, for a measured valueof the dominance rate which is intermediate between −1 and +1, thecyclopean eye CE is moved from R_(OR) towards R_(OL) proportionally tothe measured value.

When the wearer is looking at an object point OP contained in his visualenvironment, a cyclopean gaze direction G_(CE) connects the cyclopeaneye CE to the object point OP. This cyclopean gaze direction G_(CE) isidentified itself using a lowering angle α and an azimuth angle βdefined as before but with the cyclopean eye CE used as the angle apex.Then, lowering angle values and azimuth angle values are defined forboth eyes, respectively α_(R) and β_(R) for the right eye 10 and α_(L)and β_(L) for the left eye 20, corresponding to the respective gazedirections of the eyes when the wearer is looking at the object point OPthrough the lenses 1 and 2. Thus, the respective gaze directions of theeyes 10 and 20 for binocular vision are defined by the values of α and βrelating to the cyclopean gaze direction. These values are used for themaps of FIGS. 2a to 2d . According to the cyclopean coordinate system,gaze directions which are oriented downwards with respect to the fittingcross FC have positive values for the lowering angle α, and gazedirections which are oriented towards the right side according to thepoint of view of the wearer with respect to the fitting cross FC ormeridian line ML have negative values for the azimuth angle β. Bothangles α and β are zero for gaze direction G_(FC).

Each lens 1, 2 is of progressive type for presbyopia-correction. Then itproduces a mean refractive power PPO_(α,β) and a lens astigmatismAST_(α,β) which vary continuously depending on the gaze direction Gthrough the lens. So the PPO_(α,β)- and AST_(α,β)-values vary as twofunctions of the α- and β-angle values for each lens 1, 2. The resultingastigmatism ASR_(α,β) of one lens for any gaze direction G through thislens is the difference between the actual astigmatism value AST_(α,β) ofthe lens for this gaze direction and the prescribed astigmatism for thesame lens. It is also commonly called unwanted astigmatism.

For each lens 1, 2 separately from the other lens, a mean refractivepower deviation for each gaze direction G(α,β) through this lens isdefined as follows:DPPO _(α,β) =PPO _(α,β) −PPO _(α,βmeridian),where PPO_(α,βmeridian) is the mean refractive power on the meridianline ML of the lens for the value of the lowering angle α. DPPO is thuscalculated between the gaze direction G and the meridian line ML of thelens, at constant value for the lowering angle α.

A total deviation combines the mean refractive power deviation and theresulting astigmatism for any gaze direction G(α, β) through one of thelenses, as follows:P _(α,β) =m*(DPPO _(α,β))^(a) +n*(ASR _(α,β))^(b),where m and n are fixed positive coefficients which can not be zerosimultaneously, and a and b are exponents higher than zero and less thanor equal to 2. The total deviation is also defined for each lensseparately from the other one.

Then, two threshold values are defined for the total deviation based onthe prescribed addition ADD, respectively for far vision and proximatevision. The threshold value for far vision ism*(ADD/8)^(a)+n*(ADD/4)^(b), and that for proximate vision ism*(ADD/4)^(a)+n*(ADD/2)^(b).

For each one of the lenses 1 and 2 independently, two gaze directionsdenoted G₁ and G₂ are defined both with the same value for the loweringangle α as the gaze direction G_(FC), and where the total deviation isequal to the above-threshold for far vision. β₁ and β₂ are therespective azimuth angle values of the gaze directions G₁ and G₂. Theabsolute difference in the azimuth angle β between the gaze directionsG₁ and G₂, namely ABS(β₂−β₁), is a measurement of the width of the farvision field of the lens of concern, and has been called first widthvalue in the general part of the description. ABS stands for theabsolute or unsigned value. In FIG. 1c , this first width value isdenoted H_OR_VL for the right lens 1 and H_OL_VL for the left lens 2. Inthis notation, H stands for horizontal measurement direction, OR and OLstand respectively for right eye and left eye, and VL stands for farvision.

Similarly, two other gaze directions denoted G₃ and G₄ are furtherdefined for each of the lenses 1 and 2 independently. They both haveapproximately the same value for the lowering angle α as the proximatevision gaze direction G_(VP). The total deviation equals the thresholdfor proximate vision as defined above at the gaze directions G₃ and G₄.β₃ and β₄ are the respective azimuth angle values of the gaze directionsG₃ and G₄. The absolute difference in azimuth angle β between the gazedirections G₃ and G₄, ABS(β₄−β₃), is a measurement of the width of theproximate vision field of the lens of concern. It has been called secondwidth value in the general part of the description. In FIG. 1c , thissecond width value is denoted H_OR_VP for the right eye 1 and H_OL_VPfor the left eye 2. In this notation, VP stands for proximate vision.

Gaze directions G₁ and G₃ are oriented towards the nasal side of eachlens 1, 2, and gaze directions G₂ and G₄ towards their respectivetemporal side. All coordinates α₁ to α₄ and β₁ to β₄ relate to thecyclopean coordinate system.

The discrepancy in first width value between lenses 1 and 2 may bequantified using the following relative differences:Ratio_H_VL=(H_OR_VL−H_OL_VL)/(H_OR_VL+H_OL_VL) for far vision, andRatio_H_VP=(H_OR_VP−H_OL_VP)/(H_OR_VP+H_OL_VP) for proximate vision.

In addition, the following meanings can be assigned:

-   -   ABS(β₁) is the first nasal half-width for the right lens 1 or        the left lens 2, denoted H_OR_VL_N or H_OL_VL_N respectively,        and relates to far vision,    -   ABS(β₂) is the first temporal half-width for the right lens 1 or        the left lens 2, denoted H_OR_VL_T or H_OL_VL_T respectively,        and relates to far vision too,    -   ABS(β₃) is the second nasal half-width for the right lens 1 or        the left lens 2, denoted H_OR_VP_N or H_OL_VP_N, but relates to        proximate vision, and    -   ABS(β₄) is the second temporal half-width for the right lens 1        or the left lens 2, denoted H_OR_VP_T or H_OL_VP_T, and relates        to proximate vision again.

A discrepancy in the nasal and temporal half-widths can be quantifiedfor each eye using the other following relative differences:for the right lens 1 and for far vision:Ratio_H_OR_VL=(H_OR_VL_N−H_OR_VL_T)/(H_OR_VL_N+H_OR_VL_T)for the right lens 1 and for proximate vision:Ratio_H_OR_VP=(H_OR_VP_N−H_OR_VP_T)/(H_OR_VP_N+H_OR_VP_T)for the left lens 2 and for far vision:Ratio_H_OL_VL=(H_OL_VL_N−H_OL_VL_T)/(H_OL_VL_N+H_OL_VL_T)for the left lens 2 and for proximate vision:Ratio_H_OL_VP=(H_OL_VP_N−H_OL_VP_T)/(H_OL_VP_N+H_OL_VP_T)

In the invention exemplifying embodiment characterized by the maps ofFIGS. 2a to 2d , the prescribed far vision mean refractive power is −1.0diopter for both right lens 1 and left lens 2. The prescribed additionADD is 2.50 diopters for both lenses 1 and 2. This invention embodimenthas been obtained by implementing computer means, in particular for thelens optimization step. Such optimization step is performed in a mannerwell-known in the art, using at least one merit function whichquantifies a combination of deviations each between an actual value forone design parameter and a target value for this parameter. Forimplementing the invention, the total horizontal width ratio andpossibly also the relative difference in the maximum mean refractivepower gradient between both lenses may be handled each as one designparameter for the optimization step.

The formula used for the total deviation corresponds to m=0, n=1 andb=1, so that P_(α,β)=ASR_(α,β).

The threshold condition for far vision which is met for the gazedirections G₁ and G₂ through each lens 1, 2 is thereforeASR_(α,β)=ADD/4=0.625 diopter. Then from the maps of FIGS. 2c and 2d ,the respective azimuth angle values β₁ and β₂ for the gaze directions G₁and G₂ are:

-   -   for the right lens 1: β₁=22.47° towards nasal side and        β₂=−29.61° towards temporal side    -   for the left lens 2: β₁=−14.5° towards nasal side and β₂=16.78°        towards temporal side

Hence, H_OR_VL=52.08° for the right lens 1, and H_OL_VL=31.28° for theleft lens 2, and the total horizontal width ratio Ratio_H_VL of the lenspair for far vision equals 0.25 in absolute value, which is more than0.10.

The other threshold condition for proximate vision which is met for thegaze directions G₃ and G₄ through each lens 1, 2 is thereforeASR_(α,β)=ADD/2=1.25 diopters. The proximate vision gaze directionG_(VP) used is that one which crosses the meridian line ML of each lens1, 2 and corresponds to a mean refractive power equal to the prescribedfar vision mean refractive power increased by 100% of the prescribedaddition ADD. Again from the maps of FIGS. 2c and 2d , the respectiveazimuth angle values β₃ and β₄ for the gaze directions G₃ and G₄ are:

-   -   for the right lens 1: β₃=7.67° and β₄=−11.66°    -   for the left lens 2: β₃=−11.13° and β₄=15.63°

Thus, H_OR_VP=19.33° for the right lens 1, and H_OL_VP=26.76° for theleft lens 2, and the total horizontal width ratio Ratio_H_VP of the lenspair for proximate vision equals 0.16 in absolute value, which isgreater than 0.10 again.

The relative difference between the first nasal and temporal half-widthsfor the right lens 1, denoted Ratio_H_OR_VL above, equals(H_OR_VL_N−H_OR_VL_T)/(H_OR_VL_N+H_OR_VL_T)=(22.47−29.61)/(22.47+29.61)=−0.137.And, the relative difference between the second nasal and temporalhalf-widths again for the right lens 1, noted Ratio_H_OR_VP, equals(H_OR_VP_N−H_OR_VP_T)/(H_OR_VP_N+H_OR_VP_T)=(7.67−11.66)/(7.67+11.66)=−0.206.

Similarly for the left lens 2, Ratio_H_OL_VL equals(H_OL_VL_N−H_OL_VL_T)/(H_OL_VL_N+H_OL_VL_T)=(14.50−16.78)/(14.50+16.78)=−0.073,and Ratio_H_OL_VP equals(H_OL_VP_N−H_OL_VP_T)/(H_OL_VP_N+H_OL_VP_T)=(11.13−15.63)/(11.13+15.63)=−0.168.

The ratios Ratio_H_OR_VL and Ratio_H_OL_VL are thus less than 0.15, inabsolute values.

In addition for the current embodiment of the invention, the relativedifference in the first width value between both lenses 1 and 2 of thepair, namely (H_OR_VL−H_OL_VL)/(H_OR_VL+H_OL_VL), is positive.Simultaneously, the relative difference in the second width valuebetween both lenses 1 and 2 of the pair, namely(H_OR_VP−H_OL_VP)/(H_OR_VP+H_OL_VP), is negative, so that this latter isopposite in sign to the previous relative difference relating to firstwidth values.

For each one of the lenses 1 and 2 separately, a channel is defined foreach lens 1, 2 as the set of all gaze directions which are separatedfrom the line ML_min by less than 5° in the azimuth angle β according tothe cyclopean coordinate system. The side boundaries of the channel areindicated in broken lines in FIGS. 2a to 2d and the channel itselfdenoted CH.

A mean refractive power gradient is also calculated for each gazedirection G through any of the lenses 1 and 2 independently from theother lens. It is the module of the two-dimension vector with firstcomponent equal to the α-derivative of the mean refractive power, namelythe derivative of PPO_(α,β) with respect to the lowering angle α, andsecond component equal to the β-derivative of the mean refractive powerPPO_(α,β). Both derivatives are calculated for the same gaze direction Gfor obtaining the mean refractive power gradient relating to this gazedirection.

Then, a maximum mean refractive power gradient is determined for eachone of the lenses 1 and 2 separately, outside the corresponding channelCH but with an angular distance of less than 40° from the gaze directionG_(PRP). In the maps of FIGS. 2a to 2d , the circles denoted CCcorrespond to all gaze directions which are separated exactly by 40° inangular distance from the gaze direction G_(PRP). Therefore, the maximummean refractive power gradient is assessed for each one of the lenses 1and 2 inside the circle CC but outside the channel CH. It can be denotedMax_Grad_PPO_(α,β) _(_)OR for the right lens 1 and Max_Grad_PPO_(α,β)_(_)OL for the left lens 2.

For lens 1 and from FIG. 2a , the maximum mean refractive power gradientMax_Grad_PPO_(α,β) _(_)OR, is 0.125 diopter/°, produced for α=−31° andβ=6°.

For lens 2 and from FIG. 2b , the maximum mean refractive power gradientMax_Grad_PPO_(α,β) _(_)OL, is 0.113 diopter/°, produced for α=−23° andβ=6°.

Therefore, the relative difference in the maximum mean refractive powergradient between both lenses 1 and 2, namely (Max_Grad_PPO_(α,β)_(_)OR−Max_Grad_PPO_(α,β) _(_)OL/(Max_Grad_PPO_(α,β)_(_)OR+Max_Grad_PPO_(α,β) _(_)OL), equals 0.05, thus less than 0.08 inabsolute value.

A pair or progressive ophthalmic lenses according to the invention maybe designed by optimizing at least one of the lenses 1 and 2 withrespect to the total horizontal width ratio and the relative differencein the maximum mean refractive power gradient between both lenses.Furthermore, the optimizing step may take into account additional datasuch as data linked to the wearer.

In first design method implementations, the optimization step maycomprise the following substeps:

-   -   obtaining at least one data linked to the wearer;    -   based on said data linked to the wearer, determining one of the        wearer's eyes as a main eye or determining a value of a        dominance rate for both eyes;    -   obtaining a relationship between a modulation parameter for the        total horizontal width ratio and the main eye or value of the        dominance rate; and    -   performing the optimization using the modulation parameter for        the total horizontal width ratio.

Data linked to the wearer means any data related to the wearer or acombination thereof, such as data linked to at least one eye of thewearer, data linked to optometric measurements carried out on the weareror data linked to lifestyle, posture or handedness of the wearer.

In such first implementations, data linked to the eye means either oneof the following data or any combination of several of them:

-   -   prescription data such as prescribed optical power, prescribed        astigmatism with module and axis orientation, mean power also        called mean sphere and equal to the prescribed optical power        added to half of the prescribed astigmatism, prescribed prism        with value and orientation, etc;    -   data related to the high order aberrations of the eye;    -   data related to the eye's biometry such as the position of the        eye rotation center, the cornea, the pupil, or the pupil        diameter;    -   binocular data related to the binocular behavior of both eyes,        and allowing to identify one of the eyes as the main eye: which        one of the eyes is the dominant one, the sighting one, the eye        with the best acuity, the preferred eye, the eye with the        highest or lowest amount of high order aberration, the eye which        is more sensitive to blur and/or contrast, etc.

“Ocular dominance” is defined for example in the Dictionary of visualscience, 4^(th) edition, D. Cline, H W Hofstetter, J R Griffin. “Visualacuity” refers to acuteness or clearness of vision, which depends inturn on the sharpness of the retinal focus within the eye and thesensitivity of the interpretative capability of the brain. “Blursensitivity” represents the acuity loss produced by a defocus introducedin front of the eye. “Contrast sensitivity” refers to the ability todetect contrast, and may be determined by measuring the minimum contrastlevel which can be detected.

When data linked to the eye are used, one of the wearer's eyes may bedetermined as the main eye or a value of a dominance rate for both eyesmay be determined.

When prescription data are used, the main eye may be that one of theeyes which has the lowest mean power value, in absolute value.

When binocular data are used, the main eye may be the dominant eye orthat of the eyes which is more blur-sensitive or has better acuity.

Then, the lens with the largest value for the first width, namely thelens corresponding to the largest value among H_OR_VL and H_OL_VL, maybe assigned to the main eye.

The modulation parameter may be a weight parameter used for the totalhorizontal width ratio in the merit function involved in theoptimization process. Alternatively, it may be a distribution parameterof design targets.

The optimization step itself may be computer-implemented. But it mayalso involve a selection of a semi-finished blank for each eye, amongblanks having respective designs different from each other butcorresponding to the prescribed addition. Then, the face of eachselected blank opposite to that provided with the desired design ismachined so as to produce the optical power and astigmatism prescribedfor the eye of concern.

When data linked to optometric measurements are used, one of thewearer's eyes may be determined as the main eye or a value of adominance rate for both eyes may be determined.

Data linked to lifestyle of the wearer means data related to jobs andhobbies of the wearer. Different wearers, though, typically havedifferent lens usage requirements, some of which being related to thewearer's vocational and avocational activities. Different vocational andavocational activities may encompass different ranges of accommodativedemands due to, for example, variation in the working distancerequirements of the activity and the nature and relative motion of thevisual targets associated with the activity. Thus, different activitiesway warrant different considerations in the selection of an appropriatelens design. Data linked to lifestyle can be obtained by methods such asinterviews and surveys at an optometrist, a store or the like.

When data linked to lifestyle are used, one of the wearer's eyes may bedetermined as the main eye or a value of a dominance rate for both eyesmay be determined. Particularly, when data linked to lifestyle aresegmented, one of the wearer's eye may be determined as the main eye. Ifdata linked to lifestyle relate to a continuous criterion, a value of adominance rate for both eyes may be determined. Data linked to lifestyleare advantageously used in combination with other data linked to thewearer, particularly data linked to posture or handedness.

Data linked to posture of the wearer means data related to positions andorientations of body parts (head, trunk, etc.) of the wearer to performa task (reading, writing, watching computer screen, etc.) in determinedenvironmental conditions (relative positions and orientations ofobjects). From posture adopted by an individual can derive knowledge ofdistances of object seen as a function of the gaze direction (ergorama).

When data linked to posture are used, one of the wearer's eyes may bedetermined as the main eye or a value of a dominance rate for both eyesmay be determined. Particularly, when data linked to posture aresegmented, for example if an object is seen either on the left side oron the right side of the sagittal plane of the wearer, one of thewearer's eye may be determined as the main eye. If data linked toposture relate to a continuous criterion, for example the distance fromthe object to the sagittal plane, a value of a dominance rate for botheyes may be determined.

When data linked to posture are used in combination with data linked tolifestyle, for example if the wearer mainly performs near vision tasksand if his posture is such that the objects seen in near vision aremainly on the right side of his sagittal plane, then the design of theright eye lens may be wider in near vision than that of the left eyelens.

Data linked to handedness of the wearer means data related to manuallaterality, or laterality, of the wearer. Right-handed persons andleft-handed persons behave very differently when executing certain nearvision tasks. The specific near vision task of writing on a sheet ofpaper is usually considered for determining laterality. The lateralityof a person can thus be defined by the hand used for writing on a sheetof paper, or more accurately, by calculating an handedness scoreconsisting in asking questions about the hand used in everyday tasks.The Edinburgh Handedness Inventory is an example of such a way todetermine an handedness score (Oldfield R. C. (1971), “The assessmentand analysis of handedness: The Edinburgh Inventory”, Neuropsychologia,vol. 9, p. 97-113).

When data linked to handedness are used, one of the wearer's eyes may bedetermined as the main eye.

When data linked to handedness are used in combination with data linkedto lifestyle, for example if the wearer mainly performs near visiontasks and if he is right-handed, then the design of the right eye lensmay be wider in near vision than that of the left eye lens.

In second design method implementations, the optimization step maycomprise the following substeps:

-   -   obtaining target designs respectively for both lenses of the        pair;    -   based on data linked to the wearer, determining one of the        wearer's eyes as a main eye or determining a value of a        dominance rate for both eyes;    -   obtaining a relationship between a modulation parameter for the        total horizontal width ratio and the main eye or value of the        dominance rate;    -   modifying the target designs using the modulation parameter for        the total horizontal width ratio; and    -   performing the optimization using the modified target designs.

The difference between the first and second design methodimplementations relates in the use of the modulation parameter duringthe optimization process itself for the first implementation, whereas itis used to define the optimization target for the second implementation.

The invention claimed is:
 1. A method for providing a pair ofprogressive ophthalmic lenses to an identified wearer, said methodcomprising the following steps: /1/ for the wearer of both lenses:obtaining the prescriptions for both eyes including a prescribedaddition ADD for each of the two lenses, said prescribed addition beingcommon to both lenses of the pair; obtaining a value for a separationdistance between both pupils of the wearer, and a position of acyclopean eye for the wearer; and defining a cyclopean coordinate systemfor the wearer, based on the pupil separation distance and the cyclopeaneye position; /2/ for each lens of the pair separately from the otherlens: in standard as-worn conditions of the lens by the wearer,determining a meridian line, a nasal side and a temporal side on eitherside of the meridian line, respectively, a fitting cross, a prismreference point, a proximate vision gaze direction, a mean refractivepower PPO_(α,β) and a module of resulting astigmatism ASR_(α,β) for eachgaze direction through the lens and corresponding to a lowering angle αand an azimuth angle β within the cyclopean coordinate system; defininga mean refractive power deviation DPPO_(α,β) for each gaze direction as:DPPO _(α,β) =PPO _(α,β) −PPO _(α,βmeridian), where PPO_(α,βmeridian) isthe mean refractive power on the meridian line (LM) for the same valueof the lowering angle α as that of the gaze direction, defining a totaldeviation P_(α,β) for each gaze direction as:P _(α,β) =m*(DPPO _(α,β))^(a) +n*(ASR _(α,β))^(b), where m, n, a and bare parameters such that m≧0, n≦0, (m, n)≠(0, 0), 0<a≦2 and 0<b≦2,obtaining first width values as angular distances between first andsecond gaze directions respectively on the nasal and temporal sides ofthe lens, both first and second gaze directions with lowering angleequal to that of a gaze direction through the fitting cross, and eachwhere the total deviation P_(α,β) satisfies:P _(α,β) =m*(ADD/8)^(a) +n*(ADD/4)^(b); obtaining second width values asangular distances between third and fourth gaze directions respectivelyon the nasal and temporal sides of the lens, both third and fourth gazedirections with lowering angle equal to that of the proximate visiongaze direction, and each where the total deviation P_(α,β) satisfies:P _(α,β) =m*(ADD/4)^(a) +n*(ADD/2)^(b); obtaining a maximum meanrefractive power gradient produced for a gaze direction corresponding toa cyclopean gaze direction which is comprised: inside a zone centered atthe cyclopean gaze direction corresponding to gaze direction passingthrough the prism reference point and containing all cyclopean gazedirections respecting the following inequality (|α|²+|β|²)^(1/2)≦40°,and outside a central optical zone, said central optical zone comprisinga main line of cyclopean directions for which, for each angle α_(min),the resulting astigmatism reaches its minimum, the central optical zonebeing delimited on either side of the main line by cyclopean gazedirections whose azimuth angle is equal to β_(min)±5°; /3/ for the lenspair: defining a total horizontal width ratio as a relative differencebetween the respective first width values of both lenses of the pair, orbetween the respective second width values of both lenses of the pair,the relative difference between two values being a ratio of a differencebetween said values over a sum of said values; and defining a relativedifference in the maximum mean refractive power gradient between bothlenses of the pair; /4/ optimizing at least one lens of the pair so thatthe total horizontal width ratio has an absolute value greater than orequal to 0.10, and the relative difference in the maximum meanrefractive power gradient between both lenses is less than or equal to0.08 in absolute value; and /5/ manufacturing both lenses of the pair inaccordance with an optimization result of step /4/.
 2. Method accordingto claim 1, in which step /4/ comprises: obtaining at least one datalinked to the wearer; based on said data linked to the wearer,determining one of the wearer's eyes as a main eye or determining avalue of a dominance rate for both eyes; obtaining a relationshipbetween a modulation parameter for the total horizontal width ratio andsaid main eye or value of the dominance rate; and performing theoptimization using the modulation parameter for the total horizontalwidth ratio.
 3. Method according to claim 1, in which step /4/comprises: obtaining target designs respectively for both lenses of thepair; based on data linked to the wearer, determining one of thewearer's eyes as a main eye or determining a value of a dominance ratefor both eyes; obtaining a relationship between a modulation parameterfor the total horizontal width ratio and said main eye or value of thedominance rate; modifying the target designs using the modulationparameter for the total horizontal width ratio; and performing theoptimization using the modified target designs.